1. Field of the Invention
The present invention relates to a liquid crystal optical element wherein a composite material composed of liquid crystal and a polymer is interposed between a pair of substrates with electrodes, a liquid crystal display element and a projection type liquid crystal display apparatus using such element.
2. Discussion of Background
In recent years, liquid crystal displays have been widely used for personal word processors, hand-held computers, portable TV sets and so on by making use of advantages of low consumption rate of power, low voltage driving and so on. Of these liquid crystal displays, liquid crystal display elements having active elements which are excellent in viewing angle, of high speed response and capable of high density display, have particularly been noted and developed.
At the beginning, liquid crystal display elements (LCDs) of dynamic scattering type (DSM) have been proposed. However, such DSM-LCDs had a disadvantage of large current consumption because a high value of electric current passed in the liquid crystal. Now, LCDs of twisted nematic type (TN) using a polarizing plate have been widely used in markets as display elements for portable TVs or portable type information devices. Since the TN-LCDs have a very small leak current and a small power consumption rate, they are suitable for usage in which batteries are used as power sources.
In DSM-LCDs having active elements, the leak current of the liquid crystal itself is large. Accordingly, it was necessary to provide a large storage capacitance in parallel to each picture element, and the power consumption of the liquid crystal display elements themselves was large.
Since the leak current of the liquid crystal itself in the TN-LCDs is very small, it is unnecessary to provide a large storage capacitance, and the power consumption of the liquid crystal element itself can be small. However, there is a problem that the transmittance of light is small because two polarizing plates are required in the TN-LCDs. In particular, when a color filter is used to obtain a colored display, only several percents of incident light can be utilized. Accordingly, a strong power source is required, as a result of which power consumption rate is increased.
There are further problems that an extremely strong light source is required when a picture image is to be projected which causes difficulty in obtaining a high contrast ratio on a projection screen and which changes the operating condition of the liquid crystal display element due to heat from the power source.
In order to solve the above-mentioned problems, there has been proposed a liquid crystal/polymer composite material in which a nematic liquid crystal is dispersed and held in a matrix comprising polymer or the like, or a composite material comprising a continuous liquid crystal phase and a polymer in a network shape is formed. And by utilizing the scattering-transmitting characteristics, there has been obtained a liquid crystal display element capable of controlling turning-on and off of light directly without using any polarizing plate. Such liquid crystal display element is called a dispersion type liquid crystal display element or a polymer dispersion type liquid crystal display element. The basic construction and a method or producing the liquid crystal display element will be described.
A liquid crystal optical element includes a nematic liquid crystal having a positive dielectric anisotropy, and the refractive index of a polymer phase is made substantially in coincidence with the ordinary refractive index (n.sub.o) of the liquid crystal. A liquid crystal/polymer composite material is interposed between a pair of substrates with electrodes.
Each of the substrates with electrodes is a substrate made of glass, plastics, ceramics or the like on which a transparent electrode such as ITO (In.sub.2 O.sub.3 --SnO.sub.2), SnO.sub.2 or the like is formed. A metallic electrode made of material such as Cr, Al or the like may be used in combination of the above-mentioned electrode, if necessary. When the electrodes are used for a reflection type operation mode, they can be used as reflection electrodes.
In the liquid crystal/polymer composite material, the refractive index of the liquid crystal is changed depending on a state of the application of a voltage across the substrates with electrodes. When the refractive index of the polymer phase substantially agrees with the refractive index of the liquid crystal, light is transmitted, while when they do not agree with each other, light is scattered. Since the liquid crystal optical element does not use a polarizing plate, a bright display is generally obtainable.
When a voltage is applied, liquid crystal molecules are aligned in parallel to a direction of electric field. Accordingly, it is easy to control the refractive indices, and a high transmittance is obtained in a light transmission state of the liquid crystal optical element.
Conventional techniques of the liquid crystal optical element using the liquid crystal/polymer composite material will be described.
Japanese Unexamined Patent Publication No. 271233/1988 (U.S. Pat. No. 4,834,509) (referred to as a conventional technique 1) discloses that a mixture of liquid crystal and a polymer material is produced by using a vinyl compound as the polymer material, specifically, an acryloyl compound containing urethane acrylate of high molecular weight, and the mixture is subjected to a photopolymerization phase separation process to form a polymer phase and a liquid crystal phase whereby a liquid crystal/polymer composite material of high performance can be formed. The publication also discloses that a light modulator of good appearance and high performance can be obtained by controlling light passing through the layer of liquid crystal/polymer composite material with an outer electric signal turned-on and off.
In the conventional technique 1, there is a proposal that a curable material including an --OH group-containing vinyl monomer which is 2-hydroxyethylacrylate (2-HEA), i.e., a compound expressed by the formula (1) which will be described hereinbelow, where R is an ethylene group, is used for the liquid crystal/polymer composite material. Formation of a cured product of a curable material comprising an --OH group-containing vinyl monomer as a polymer phase was an important technical factor in adjusting the interaction of contact at the interface between the liquid crystal phase and the polymer phase in the liquid crystal/polymer composite material so as to form a suitable phase separation structure.
Japanese Unexamined Patent Publication No. 196229/1986 (referred to as a conventional technique 2) discloses as general description a liquid crystal display element formed by combining a liquid crystal/polymer composite material layer comprising polymer and liquid crystal with an active matrix substrate.
The active matrix substrate is composed of a substrate on which electrodes and active elements such as thin film transistors (TFTs), thin film diodes, metal-insulation material-metal non-linear resistor devices (MIMs) or the like are formed. A single or a plurality of active elements are connected to each of the picture element electrodes. The counter electrode substrate is composed of a substrate on which a common electrode or a patterned electrode is formed, and the counter electrode substrate is combined with the active matrix substrate so as to be capable of providing a display.
In a case of using a three-terminal element such as TFT as the active element, a solid electrode used in common with all picture elements may be disposed on the counter electrode substrate. In a case of using a two-terminal element such as an MIM element or a PIN diode or the like, the counter electrode substrate is applied with a stripe-like patterning.
Further, as disclosed in Japanese Unexamined Patent Publication No. 33523/1989 (conventional technique 3), when a liquid crystal/polymer composite material is formed by photopolymerization, liquid crystal in the liquid crystal/polymer composite material is controlled to have a direction of orientation by applying an electric field from the outside whereby a normally transparent portion or a semi-transparent portion is previously formed. When a fixedly displayed portion is desirable, such normally transparent portion can be formed.
In the initially developed liquid crystal display elements provided with such liquid crystal/polymer composite materials as described in the conventional techniques 1 and 2, there was hysteresis in the voltage-transmission characteristics (V-T curves) in the electro-optical characteristics of the liquid crystal display elements. Although the hysteresis is negligible in a window or a shutter which is operable under the condition of applying two values, there was a problem that the transmission of light varies between a state that a driving voltage rises and a state that the driving voltage falls in a display element of high performance which requires to display a half tone. Accordingly, there was an image-sticking phenomenon wherein a picture image which had appeared in the display just before the changing of picture remained in the present display for several seconds.
In consideration of such problem, Japanese Unexamined Patent Publication No. 186535/1994 (U.S. Pat. No. 5,196,952) (referred to as a conventional technique 4) presents a propose concerning the physical values of a liquid crystal material and control of the structure of a polymer phase as a result of having paid attention to the physical values of the liquid crystal used and the spatial dispersion of liquid crystal domains in a liquid crystal cell, and discloses that the reduction of hysteresis to a required extent in a display element has been achieved. For instance, there is description that a combination of a refractive index anisotropy .DELTA.n of liquid crystal of 0.18 or more and a dielectric anisotropy .DELTA..epsilon. of 5-13 is preferable. Also, there is description that the shape of liquid crystal domains with a certain deformation and a random arrangement of liquid crystal in the liquid crystal cell contribute to the reduction of the hysteresis.
Japanese Unexamined Patent Publication No. 134238/1993 (U.S. Pat. No. 5,235,445) (referred to as a conventional technique 5) discloses that in considering the elasticity of a polymer phase used, a polymer material having an elasticity of 3.times.10.sup.7 N/m.sup.2 or lower at 20.degree. C. and 1.times.10.sup.3 N/m.sup.2 or more at 40.degree. C. should be used. It also describes that control of the elasticity of the polymer phase contributes greatly to the reduction of the hysteresis, and a beautiful display without an image-sticking can be obtained even when a moving picture is to be displayed.
In the conventional liquid crystal/polymer composite materials, however, there is a large temperature dependence in the voltage/transmission characteristics. In particular, the scattering power of the liquid crystal optical element decreases when the ambient temperature is in a low temperature range of 20.degree. C. or less, and the hysteresis which is expressed by a difference of transmission between a voltage rise and a voltage fall in the voltage-transmission characteristics becomes large with the result of the problems that the contrast ratio of a displayed picture decreases and there results an image-sticking wherein a picture image which had appeared in the display just before the changing of picture remained in the present display for several seconds.